| EXAMPLE FLIGHT by Dave Bitzer |
| Celestial Navigation by sextant was a lot of paperwork and table lookups. To ease the workload at the beginning of your virtual sextant experience, I have included the data for an example flight. In real weather, if the headwind component exceeds 25 knots, you may not have enough fuel capacity to fly the whole distance non-stop. I usually fly it in fair weather because this is a sextant test, not a range test. Of course you may select or design your own flights if and when you feel ready to do so. That is the whole idea. This is the route (for which DC-3 Airways pilots can earn credit): AA_113R St Paul to Felts (KSTP to KSFF) The MS Flight planner says: 1019 nm, 625 gallons fuel, 6:51 elapsed time (no headwind) 19 April, 1945 is the selected date Fair Weather, Winds aloft, from the west at 22 knots Cruise altitude, 12,000 ft. Sunrise at the destination 1315Z OAT = -9 degrees C at 12,000 ft. The flight plan calls for a night flight for sextant navigation, with a daylight landing for safety. The plan below therefore was developed backwards from selecting an arrival time at the airport of 1340Z (about 1/2 hour after sunrise). This gives us a sun line of position at the end, if needed. Using the KSFF approach plate for NDB RWY 3L, we see a transition waypoint at KARPS, and an IAF at SFF. Take off is therefore scheduled for 0555Z. Care was taken to select sun shots after 0600 Local Time, since the Sextant is pre-programmed to switch from stars to sun at that time. Using the FS9 Flight planner, select waypoints about 250 nm apart. Use a 20 knots headwind, which is what is forecast, and use the average of the start and end Magnetic Headings for each leg, because the variation changes fast during this flight. You are expected to adjust the heading on each leg to counter any cross winds, and adjust the power setting to get to the next waypoint on schedule. This isn't a tutorial on how to fly the route, but it impacts the pre-calculated star data if you are not on time. The important thing is taking the shots at the prescribed times, which is where you know the position of the stars. Generally, you may be off up to 200 nm without becoming inaccurate with this form of celestial navigation, which is over an hour of accumulated time error at DC-3 speeds. I assumed there are no radio or light beacons en-route, but it is ok to use the NDB at SFF as an E/W LOP to cross with the Sun's North/South LOP on approach. (If there weren't an NDB there at that time, there would have been tower voice radio, or a commercial broadcast station that could be used as a short distance homing signal instead.) As a caution, there are several airports in the vicinity of the destination, so take advantage of the SFF NDB at the airport. You may safely descend to 8500 ft when you are 28 NM east of Karp, and further descend to 6600 ft west of Karp. If you set up the sextant for Karp, you can use it as a speed line to determine when SOD should occur. The table below shows some of the FS Planner data. I have put these tables on a page so you can print this page out as an in-flight reference, and fill in the blanks during the flight. In the table below, column headings are: N-d = The degrees North part of the waypoint latitude N-m = The minutes part of the waypoint latitude W-d = The degrees West part of the waypoint longitude. W-m = The minutes part of the waypoint longitude Mag = The no wind average magnetic heading to that waypoint from the previous. Zulu = The ETA at that waypoint in GMT Mag-va r = The game's magnetic variation at the waypoint in degrees east of north. El-d = The degrees part of the angle from bubble horizon to the star Az = The azimuth in degrees east of true north to the star Wheels up at 0555Z, climbing to 12,000 ft, on oxygen as needed. |
| To complete the Mag-var data, it is 3 deg at KSTP, and 19 deg at SFF and KSFF. Using USNO data, use Polaris for the course line, and a suitable second star for the speed line. We get the elevations and azimuths for the specified times below: |
| Shortly after reaching cruise altitude, and completing your checklist, start setting up the sextant for the first star shot. About 3 minutes before Zulu shot time, start the 1 minute measurement of the star that will give you your course line. In this case it is Polaris. This line will not change much in the next 4 minutes. As soon as you get that distance, enter it in the (in this case) Star 2 column, change the worksheet star elevation and azimuth to the other star, to get the speed line (in this case Polaris), and start its 1 minute average 1/2 minute before Zulu shot time. Then you can start plotting within a minute of shot time and make any necessary corrections in heading or speed promptly. Repeat this process as you approach each successive waypoint. Follow the steps in section 1 above to set up the worksheet, and record the measured distance on the table above. Immediately plot these LOPs, and determine the actual position as an intersection on your chart. Use of real sectionals to plot on is nice (in pencil only), or you may use some cross hatch paper. At this latitude, it is convenient to use 6 squares per degree of latitude, and 4 squares per degree of longitude to plot your position. Make the necessary correction to steer to the next waypoint. My rule of thumb is that one degree of correction will move the plane 2 nm every 100 nm, for small corrections. Once I am set up at cruise for the next waypoint, I usually go to 4x or 8x speed, just to speed up the process, slowing again to normal speed when the time for the next shot pair approaches. For your convenience I have included the three files comprising the flight above. If you paste them into your "My Documents\Flight Simulator Files" folder, it might save you some time. The aircraft I have modified for sextant testing is titled "Douglas DC-3 Paint 2" You can edit the AA_113R.FLT file to change the aircraft to your modified aircraft's title, or just call up that flight, and switch to your aircraft and resave it. Since the sample flight plan uses a 20 knots headwind, I have also included the weather (fair, theme 149) file so we are working off the same sheet. Of course, you should later on use real weather, winds aloft enabled. NOTE: In the [flightsim.x] portion of the aircraft.cfg for a particular FS9 aircraft, two lines control aircraft titles. First is the "title=" line, and that title is used by the *.flt file to specify an aircraft. Then there is the "ui_variation=" line, and that title is displayed by the pull down aircraft menu for manually specifying the aircraft you want to use. |